but SBD is debt.

DAI is literally debt! What do you think the "D" in CDP stands for?

You lock up ETH and are lent (i.e. debt) DAI as long as you leave the ETH locked up. In order to repay the loan and get your ETH collateral back, you have to return the DAI.

ETH must fail really hard to crash 90%.

I don't really agree with this. Bitcoin has dropped 90%, even 95%, and didn't fail. ETH dropped over 90% (almost 95%?) from December 2017 to December 2018 and that wasn't considered failing; it was just a price drop. It is just the nature of highly volatile crypto assets that big drops are possible (as are big increases). It's not desired to see that kind of drop but is almost expected, and it has to be considered in analyzing something like DAI.

I don't mean to say that DAI would certainly fail catastrophically in the case of a big drop in ETH (and/or decline in organic demand for DAI itself) but then again SBD didn't entirely fail either despite STEEM dropping roughly 99%! It suffered a temporary haircut period but has now mostly recovered.

So overall I would say we are still in an experimental period of figuring out how these systems should work best. You might also be interested to read this critique of DAI as not being scalable. (I would not claim that SDB is actually any better.)